I think you should check your power line environment, this may be helpful
http://www.tp-link.com/en/faq-409.html

Hayden, with a physical line bandwidth of 120 Mbit/s, getting a payload throughput of about 40 is realistic and normal. Just like any other shared-media network (e.g. WLAN), actual throughput is about 30 to 40 percent of physical bandwidth.

So what you should be working on is physical bandwidth. Follow that FAQ to make the best of what your house brings.

Make sure you are a) running the latest firmware and b) plugging directly in wall sockets and not using extension leads. Also try different sockets if that's an option.